This invention relates generally to the control of various types of closed loop systems which receive input commands and execute such commands in order to achieve an output level commanded by the input. More particularly, the invention is directed to an improved control circuit for accurately maintaining the output at the commanded level.
Such command systems have wide application throughout a variety of industries and different technologies. For example, the speed of electric motors is controlled at selected levels in a large number of motor driven machines, including products as diverse as machine tools, wheelchairs and other electrical equipment both large and small. Other examples are heating and/or cooling systems in which a selected temperature is to be maintained, hydraulic and pneumatic control systems, servo-driven machine slides or work station components which control the position of various devices, and chemical and industrial processing facilities in which parameters such as pressure, speed, force, flow rate, temperature, the rate of heat application or extraction, or the proportions involved in mixing operations are controlled.
In all of these systems, an input command is executed in order to achieve a system output which corresponds to the input command. In a motor speed control system, a speed setting dial or other input device is used to select the desired operating speed, and the motor responds to the input command by adjusting its output speed to conform with the speed which is commanded. This type of control system is generally referred to as a closed loop system which uses a feedback signal to control the motor operation in a manner to achieve coincidence of the output speed with the commanded speed. Thermal control systems such as ovens sense the temperature and turn heating elements on and off depending upon whether or not the actual temperature is above or below the selected temperature.
One of the principal problems with all closed loop command systems is that there is a "dead band" within which the actual output can fluctuate without the system responding. Thus, the output can vary from the input command within the dead band range, and the accuracy of the control system suffers accordingly. In addition, the control system typically includes mechanical components which detract from the reliability of the system and are subject to breakdowns and other mechanical malfunctions. Conventional controllers are also subject to a torque reduction when operating at low speeds.